Conventionally, a contrast method or a total reflection method is used to acquire an image that visualizes a contact area. The acquired image is then binarized using an intensity threshold value to extract the real contact area. When a contrast method is used to visualize the real contact area, materials to be visualized are limited and the accuracy in extracting a true contact region is poor. On the other hand, in the total reflection method, since a prism is used to observe a contact area obliquely, the aspect ratio of an image is not one and the contact area cannot be brought into focus very well. In the conventional extracting method, the image acquired by using either of the methods described above undergoes (1) binarization by visually comparing the acquired image with the original image to extract an image. In this case, the threshold setting and hence the extraction result differ person to person, and it takes a long time to carry out the whole procedure. (2) In the image analysis field, an Otsu method is frequently used to determine the threshold, but the method is originally directed to typical images and used to extract an object from the background. The principle according to which the Otsu method extracts an object uses the fact that the threshold is present in the valley between adjacent peaks in an intensity histogram. There is no obvious reason for directly applying the method to the problems described above, and the method is not applicable or accurate.
FIG. 14 shows the relationship between an intensity histogram and its relative threshold for binarization. Each of the left raised portions represents a histogram for a real contact area and the vicinity thereof. In a conventional method, since the region having intensity smaller than or equal to a threshold is extracted as a real contact area, it is difficult to rationally determine the threshold.
On the other hand, a white polarized light interferometry system is proposed in Japanese Patent No. 3,718,837 to acquire a contact area image viewed from the front. In this system, the real contact area can be accurately visualized, theoretically and experimentally, based on the relationship between a clearance and intensity using the acquired image along with an RGB-AND method. The RGB-AND method does not require an exact threshold, which allows for a real contact area to be extracted. That is, the intensity values of an interference image formed using the three RGB colors are simultaneously evaluated, and regions in which a real contact area has an achromatic color, the RGB intensity values have the same value, are extracted by using rough thresholds. A real contact area is eventually determined by performing logical product (AND) operation on the extracted regions. That is, when two-beam interferometry is used, since a real contact area (0-th interference fringe) has an achromatic color, the RGB intensity values have the same value, as shown in FIG. 15. Using this fact, one can perform logical product (AND) operation on the three RGB images binarized by the rough thresholds to extract a real area of contact.
On the other hand, when a tangential force is applied to a real contact area between two frictional surfaces that are made of plastic materials and in contact with each other, the real contact area grows. When the two surfaces are made of elastic materials, it is believed that the ratio of a “slip region” to a “stick region” (see FIG. 16) changes, and that the entire real contact area eventually becomes the “slip region” and undergoes macroscopic slippage (see Non-patent Document 1: Takashi Shibamiya, Masao Eguchi, and Takashi Yamamoto, “Measurement of real area of contact using intensity of white light interferometry,” Japanese Society of Tribologists, Proceedings of Tribology Conference, (May, 2008, Tokyo), page 11-12).
Further, a correlation method to identify and visualize stick and slip regions between contact surfaces in a slipping contact state are proposed (see Non-patent Document 2: Liu Jun, Kohtaro Ohba, Koji Kato, and Hikaru Inooka, “Partial slip visualization at contact surface with the correlation method”, Journal of the Visualization Society of Japan, 15, 57 (1995), pp. 133-139). Also, particle tracking velocimetry (PTV) are used to identify and visualize stick and slip regions between contact surfaces in a rolling-slipping contact state (see Non-patent Document 3: Tomoaki Iwai, Kouki Hasegawa, Seiichi Ueda, and Yoshitaka Uchiyama, “A Study on the Rolling-Sliding Friction of Rubber and the Slip in Contact Area,” Tribologists, 50, 8 (2005), pp. 620-627).
In the methods described above, which use particle image velocimetry (PIV) in a broad sense, one of the objects has a tracer (tracking marker) and the contact surfaces between them is visualized in a certain method (for example, using the fact that a real contact surface has different contrast). By successively forming only the visualized images, the shift in the marker position between two images are detected so that the stick and slip contact states can be analyzed and visualized.
Alternatively, the detection of “slippage” is carried out by preparing an acceleration sensor or a quartz oscillator and measuring the change in the signal therefrom. (see Non-patent Document 4: Shigenobu Muraoka, “Sensing Slip and Its Direction Using Quartz Resonators,” Transactions of the Society of Instrument and Control Engineers, 36, 8 (2000), pp. 639-644)
However, the above-described methods are problematic in that intensity values of the RGB three elements and the thresholds therefor, even though the thresholds can be rough values, are required and that the result tends to change depending on how well the thresholds can be set. Further, such exact/approximate thresholds are affected by illumination at the time of measurement, the material of an object under test, and characteristics of a camera used in the measurement and the presence of the thresholds disadvantageously requires calibration for each combination thereof.
Further, there are also the following other disadvantages: (1) since a contrast method is used to visualize a contact surface in conventional observation methods, the material of an object under test is limited and the accuracy in extracting a real contact region is poor, (2) it is necessary to attach a tracking marker onto a surface to be observed, (3) it is also necessary in PIV-based analysis to set an observation window whose side ranges from several pixels to several tens of pixels, and (4) a large amount of computation in PIV requires a high-speed computer.
Alternatively, to detect “slippage” by preparing an acceleration sensor or a quartz oscillator and measuring the change in the signal therefrom, a space for placing the sensor or the oscillator, variation in sensitivity depending on the location of the sensor or the oscillator, and other variety of problems need to be solved.
The present invention is proposed in view of the above aforementioned problems.
The present invention provides a novel contact area measuring apparatus and method for visualizing the contact area between two surfaces in contact with each other and extracting the real contact area from the visualized image without requiring a threshold for binarization. Further, the present invention provides a novel apparatus and method for measuring and visualizing the distribution of a stick region and a slip region in a real contact area using an intensity histogram.